Retinal mean circulation time and segmental flow will be measured in monkeys and in normal human fundi under normal and different physiological and pharmacological stimuli. Data will be collected from patients with various systemic and ocular diseases. A new technique will be used for this purpose. It consists of multiple injections of small amounts of fluorescein and continuous on-line recording of arterial and venous dilution curves by means of a highly sensitive Two-Point fluorophotometer. Using a fundus reflectometer, dilution curves of diffusible fluorescein dye and nondiffusible indocyanine green will be recorded simultaneously from various sites of the optic nerve. From the difference between the time-course of both dyes, the diffusion of fluorescein into the optic nerve will be measured and used as a test of the integrity of the blood-brain barrier. These measurements will be performed in normal subjects, in subjects with ocular hypertension, glaucoma, diabetes, and systemic hypertension. Data will be obtained also from monkeys under normal conditions, under elevated intraocular pressure, and under conditions of experimentally induced glaucoma.